This invention relates to a method of and apparatus for aspirating and dispensing small volumes of liquid.
With the increasing sophistication of analytical instruments and the ability to precisely measure or detect small quantities of material, the need for accurate dispensing of liquid samples is vitally important. This is particularly relevant when performing clinical analyses where only small quantities of sample are available. Typical sample volumes in the order of several microliters are becoming common in clinical analysis. The problem with small samples arises from the fact that if small quantities of material or sample adhere to either the interior or exterior of the dispensing tube, such quantities can represent a significant portion of the sample to be dispensed. This creates large errors in quantities dispensed. The residual sample also represents a source of contamination to the subsequent sample.
The major problem associated with metering small quantities of liquid is the inability to successfully deliver such quantities in a reproducible manner. During the process of liquid sample dispensing, the volume of a liquid drop, which may be on the order of 50 microliters, is 10 to 100 times the volume of sample required for some analyses. Consequently the process of liquid dispensing must be precisely controlled to insure adequate dispensing precision, i.e., the ability to dispense exactly the same volume of a sample each time.
Various approaches have been taken to improve liquid dispensing. One such approach, described by Reichler et al., in U.S. Pat. No. 4,121,466, is the placement of an immiscible fluid upon the outer surface of the aspirating probe of a dispensing system. Reichler uses a coaxial liquid dispenser to flow an immiscible liquid e.g., silicone oil, over the surface of the aspirating probe. Such liquid selectively wets the interior and exterior surfaces of the probe and the interior surfaces of the conduit system, to prevent the deposit of aqueous sample residues upon such surfaces. The immiscible fluid is dispensed, along with an air segment, between aspirations of successive samples, each dispensed air and sample segment being encapsulated by such immiscible fluid while passing through such system. The disadvantage of this system is the contamination of the sample mixture with the "immiscible" fluid and the inability of the system to precisely control the liquid drop formation and dispensed volume.
U.S. Pat. No. 4,259,291, issued to Smythe, describes a system which improves upon the Reichler et al. system by directly applying the immiscible fluid to the surface of the probe in a uniform and precisely controlled manner. This provides for the delivery of small volume aliquots of aqueous fluid, but once again suffers from the contamination problem.
A different approach to sample delivery is disclosed in a patent issued to Chen, U.S. Pat. No. 3,869,068. Here Chen utilizes an inner needle coaxially disposed in an outer conduit. The sample is dispensed through the inner needle and then a diluent is dispensed through both the inner needle and outer conduit. This accomplishes rapid and convenient liquid dispensing but cannot insure precise, reproducible sample delivery or dilutions since the sample aspiration is not controlled as to wetting of the outer surface of the inner needle.
Toshiba use a syringe pipette on their TBA580 Clinical Analyzer. The syringe tip is a metal needle. With this system the needle is washed after obtaining the sample liquid. Each aliquot is washed into the respective reaction/analysis chamber. The wash is water which is squirted at the sample needle by a separate flush needle (non-contacting, non-coaxial). This system suffers from the large amount of flush water required and the imprecision due to the water that remains on the sample and flush needles after dispensing.